Survival of Listeria in processed meat and poultry products

Listeria monocytogenes: 

behaviour in foods 

Tom Ross 

Food Safety Centre, School of Agricultural Science 

University of Tasmania

overview 

• some more epidemiology 

– dose-response relationships 

– susceptibility 

• eco-physiology in foods/factories 

– growth rates 

– growth limits 

– effect of lactic acid bacteria 

– the Hurdle Concept 

• predicting growth and growth limits under multiple 

environmental constraints 

Listeria Summit, Sydney, 2010

epidemiology

epidemiology 

• 85% of cases have known predisposing factors 

• very young, old, pregnant, immunocompromised are most 

susceptible 

• 3 - 10 cases per million per annum in developed nations (Oz: 60 

- 80 cases/year) 

• 25-35% mortality 

• virtually all cases believed to be food-borne 


elative susceptibility 

Susceptibility relative to 

healthy young adult 

Transplant recipients 2,584 

AIDS patients 865 

Pregnant woman 10 - 40 

Perinatal and neonates 839 

Dialysis 476 

Cancer 70 to 110 

Over 65 years 7.5 


changing epidemiology in Australia 


“infectious dose”

dose-response 

• methods of estimation 

– no ‘volunteer’ trials 

– animals models (rats, pregnant rhesus monkeys, pregnant guinea pigs) 

– epidemiology 

– “reverse” exposure assessment 

• infectious dose varies 

– by consumer 

– by strain 

– by cell ‘history’ (food type, food processing) 


dose-response (modelled) 

• FAO/WHO risk assessment: 

– * ID 50 > 10 9 cells (transplant patients) i.e. >10 7 cfu.g -1 in a 100 g meal 

– * ID one in a million > 1,600 L. monocytogenes cells (transplant patients) 

– * ID 50 > 10 12 cells (non-susceptible consumer) i.e. >10 10 cfu.g -1 in a 100 g 

meal 

• combination of strain virulence and host susceptibility means infectious dose can vary one 

million-fold 


dose-response (pregnant animals) 

• Smith et al. (2003, 2008) - pregnant monkeys, virulent strains 

– LD 50 lower = 3.63 x 10 6 , LD 50 upper = 4.27 x 10 8 

• Williams et al. (2007) - pregnant guinea pigs 

– LD 50 lower = 3.52 x 10 6 , LD 50 upper = 1.13 x 10 8 

– (n.b guinea pigs have been identified as the small animal of choice for L. monocytogenes studies 

because of the E-cadherin molecule which interacts with the internalin protein of L. 

monocytogenes) 


Dose response of L. monocytogenes-induced foetal mortality in guinea pigs and 

nonhuman primates. (Williams et al., Risk Analysis, 29: 2009).

the problem of listeriosis

the problem of listeriosis 

• very common in wet environments 

• difficult to eliminate from RTE foods*: many RTE foods 

(infrequently) contaminated 

• grows in long shelf life, refrigerated, ready-to-eat foods even 

when properly packaged and stored 

• probability of illness increases with concentration, and 

frequency, of contamination (i.e. with increased shelf life, 

reduced preservative use) 


excitement about internalins 

• internalins are essential for Listeria virulence 

• several groups reported that in some avirulent (non-pathogenic) 

mutants, there are ‘stop’ codons half-way along the internalin 

gene 

• suggests that specific strains of L. monocytogenes aren’t a 

public health threat 

• the mutation can easily be detected (with PCR) 

• inference: some L. monocytogenes in foods can ignored 


excitement about internalins 

• internalins are essential for Listeria virulence 

• several groups reported that in some avirulent (non-pathogenic) 

mutants, there are ‘stop’ codons half-way along the internalin 

gene 

• suggests that specific strains of L. monocytogenes aren’t a 

public health threat 

• the mutation can easily be detected (with PCR) 

• inference: some L. monocytogenes in foods can ignored 

• not that easy - there are many internalins (see Liu et al. 2007, Int 

J Food Micro, 118:101-115) 


excitement about ‘lineages’ 

• three genetic lineages discernable 

• lineage 1 most often involved in human illness, most outbreaks 

• lineage 1 (serotypes: 1/2b, 3b, 4b, c, d) 

• most strains in food are lineage 2 

• suggests that specific strains of L. monocytogenes are less of a 

health threat 

• “all L. monocytogenes … should be regarded as potentially 

pathogenic” (EFSA, 2007) 


eco-physiology

ecology 

• asymptomatic carriage in humans, animals 

• is essentially a saprophyte, thus… 

• survives, grows in moist and cool environments - natural or manmade, 

with pre-formed nutrients 

• colonises factories from whence it contaminates foods after 

processing 

• can grow in hard-to-access niches 

• can form biofilms that enhance resistance to cleaning (physical, 

chemical)

growth limits 

• aerobic, facultatively anaerobic 

• temperature growth range -2 to 45°C 

• readily killed by heat (not unusually heat tolerant) 

• salt tolerant 

• up to 600MPa hydrostatic pressure 


approximate doubling times 

Temperature (°C) no ‘preservatives’ “typical” processed 

meat 

( e.g sliced ham) 

35 45 min 1.5 h 

25 90 min 3 h 

15 4 h 9 h 

10 8 h 17 h 

6 17 h 34 h 

4 28 h 61 h 

2 60 h 120 h 


growth limits 

• salt (a w range 0.92 - 0.999 = up to 12-13% NaCl) 

• pH range (4.3 - > 7 (to 9.5)); less in the presence of organic acids 

• lactic acid (up to ~4mM, undissociated) 

• acetic acid (up to ~5mM, undissociated) 

• nitrite (up to 350 ppm) 

• phenol (32 ppm) 


The Hurdle Effect 

• combined factors reduce growth limits for any one factor 


The Hurdle Effect 


as a rule of thumb …. 

• if vegetative bacterial cells aren’t growing, 

they are dying …. 

• but rates of (non-thermal) death are slow and 

much slower at chill temperatures 


Lactates and Diacetates 

• much research 

– broths 

– challenge studies in product with “natural” microbiota 

• increases lag, slows growth rate 

• may prevent growth at 4°C, but less certain at higher 

temperatures 

• effect greatly enhanced at pH < 6 


Role of Lactic Acid Bacteria 

• can be an additional hurdle 

• “Jameson” effect 

10.0 

9.0 

8.0 

7.0 

6.0 

5.0 

4.0 

3.0 

2.0 

1.0 

0.0 

0 10 20 30 40 50 60 70 80 90 100 110 

lactic acid bacteria 

total count 

L. monocytogenes 

Time (days) Listeria Summit, Sydney, 2010

10 

9 

8 


7 

6 

5 

4 


3 

2 

1 

0 

0 10 20 30 40 50 60 70 

time (days) 

10 

9 

8 


7 

6 

5 

4 

3 

2 


1 

0 

0 10 20 30 40 50 60 70 

time (days)

Growth in processed chicken product in the 

absence of lactic acid bacteria 

10.0 

9.0 

8.0 

7.0 

6.0 

5.0 

4.0 

3.0 

2.0 

1.0 

0.0 


total count 


0 10 20 30 40 50 60 70 80 

Time (days)

predicting growth rates and formulations 

Mejholm and Dalgaard (2007). Modeling and predicting the growth 

boundary of L. monocytogenes in lightly preserved seafoods. Journal of 

Food Protection, 70: 70-84. 

predictor variables: 

pH, water activity, lactate, diacetate, nitrite, smoke 

compounds, temperature, lactic acid bacteria 

internet accessible: 

www.dfu.min.dk/micro/sssp/ 

Meeting details.


summary 

• despite variability between strains, L. monocytogenes has a relatively 

high “infectious dose” 

• many authorities now consider that a dose of up to 10000 cells, or 

100cfu/g, is acceptable risk, even for the most susceptible members of 

the community 

• tools, technologies are available to 

– assess the risk from various products and 

– to limit growth, and to minimise contamination 

in a rational way so that an increased tolerance can be managed to 

improve public health 


useful reviews 

• FAO/WHO (Food and Agriculture Organisation/World Health Organisation of the 

UN) (2004). 

• ILSI (International Life Sciences Institute) (2005). Achieving Continuous 

Improvement in Reductions in Listeriosis — A Risk-Based Approach. Journal of 

Food Protection, 68:1932–1994. 

• EFSA (European Food Safety Authority). (2007). Request for updating the 

former SCVPH opinion on Listeria monocytogenes risk related to ready-to-eat 

foods and scientific advice on different levels of Listeria monocytogenes in 

ready-to-eat foods and the related risk for human illness. The EFSA Journal, 

599:1-42 


acknowledgements 

• Ian Jenson, Meat and Livestock Australia 

• Food and Agriculture and World Health Organisations 

• New South Wales Food Authority 


thank you for your attention

Survival of Listeria in processed meat and poultry products

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?